Many semiconductor parts are conventionally tested using scan-based test patterns driven by automated test equipment (ATE). The ATE is typically an expensive system located in a laboratory testing environment, and use of the ATE typically involves appreciable time and resources. It can often be difficult for semiconductor designers to get sufficient access to the ATE to perform more than a minimal amount of testing, and there can be resistance to non-ATE personnel contributing to scan patterns. For example, once a part has finished its ATE testing, it can be impractical to retest semiconductors after minor changes have been made to the semiconductor parts, after scan patterns have been improved, etc.
Further, because the ATE is implemented as specialized laboratory equipment, it cannot generally be used to test semiconductors in their operational environments. This can lead to sub-optimal design and testing prior to release (e.g., over-engineering, etc.), and also tends to prevent any subsequent scan-based testing of the semiconductor parts after installation. Due to these and other ATE-related concerns, the semiconductor industry has tended to compromise between test cost and test quality, and to compromise during post-silicon bring-up and production ramp between cost for replacing parts (e.g., removal of device tested by an older test flow and attachment of devices that have been tested by a subsequent, better test flow; test flow being comprised of a sequence of test programs each associated with an environment of voltages, temperature, etc., and test program comprised of a sequence of test vectors and/or test patterns, and test pattern being comprised a sequence of test vectors, test patterns often ATE scan patterns) and enduring with parts that have potential latent problems; additionally the desire for retest can arise because of stresses incurred during the remaining manufacturing process and delivery to customers. In some cases, this has driven use of alternative testing techniques, such as so-called Joint Test Action Group (JTAG) testing, but those alternatives tend to be exceedingly slow, incomplete, or otherwise undesirable.